1. Field of the Invention
Embodiments of the invention generally relate to apparatus and methods for processing semiconductor substrates. More particularly, embodiments of the present invention relate to apparatus and methods for centering a substrate in a process chamber.
2. Description of the Related Art
During a chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD), it is desirable to have a uniform thickness profile across a substrate and no deposition near an edge area of the substrate. The area near the edge where deposition is not desired is generally referred to as the “edge exclusion.” FIG. 1A is a partial cross-sectional view showing the desired profile for a deposition layer 102 formed on a substrate 101. The deposition layer 102 is uniformly deposited across a top surface of the substrate 101 and there is no deposition within an edge exclusion area 103. Unfortunately, an actual deposition profile generally differs from the ideal configuration illustrated in FIG. 1A. FIG. 1B is a partial cross-sectional view showing an actual surface profile of a deposition layer 102a on the substrate 101 after having undergone a CVD or PECVD deposition. The deposition layer 102a typically extends to the edge exclusion area 103, and a bevel edge 104 with extra thickness may be formed near the edge exclusion area 103.
To prevent the formation of the deposition film at the edge of the substrate, FIG. 1C is a partial cross-sectional view showing one conventional approach that proposes to use a shadow ring 105. The shadow ring 105 is usually arranged at a location that overlaps and covers at least one portion of the edge exclusion area 103 of the substrate 101. As a result, as shown in FIG. 1C, a deposition layer 102b gradually reduces under the shadow of the shadow ring 105.
While the use of the shadow ring 105 can currently achieve thickness uniformity up to a 3.5 mm-wide edge exclusion area, the requirement for thickness non-uniformity has to be reduced to a 2 mm-wide edge exclusion area owing to increasingly shrinking device dimensions. As a result of the smaller edge exclusion area, a more accurate placement of the substrate 101 is required. In particular, regarding the approach using the shadow ring 105, the substrate 101 have to be placed substantially centered on the center axis of the shadow ring 105 so that the edge exclusion area 103 is correctly covered.
Therefore, there is a need for a mechanism that can center the position of the substrate in the process chamber in a cost effective and accurate manner.